Traction drive power transmissions, which transmit power to a driven part through a traction drive mechanism, have attracted attention in the field of automobiles and industrial machinery, and in recent years research and development thereon has progressed. The traction drive mechanism is a power transmitting mechanism using a rolling friction. Unlike conventional drive mechanisms it does not use any gears, which enables a reduction in vibration and noise as well as a smooth speed change in high-speed rotation. An important goal in the automobile industry is improvement in the fuel economy of automobiles. It has been suggested that if the traction drive is applied to the transmission of automobiles to convert the transmission to a continuous variable-speed transmission the fuel consumption can be reduced by 20% or more compared to conventional transmission systems since the drive can always be in the optimum speed ratio. Recent studies have resulted in the development of materials having high fatigue resistance and in the theoretical analysis of traction mechanisms. Regarding the traction fluid, the correlation of traction coefficients is gradually being understood on a level of the molecular structure of the components. The term "traction coefficient" as used herein is defined as the ratio of the tractional force which is caused by slipping at the contact points between rotators which are in contact with each other in a power transmission of the rolling friction type to the normal load.
The traction fluid is required to be comprised of a lubricating oil having a high traction coefficient. It has been confirmed that a traction fluid possessing a molecular structure having a naphthene ring exhibits a high performance. "Santotrack.RTM." manufactured by the Monsanto Chemical Company is widely known as a commercially available traction fluid. Japanese Patent Publication No. 35763/1972 discloses di(cyclohexyl)alkane and dicyclohexane as traction fluids having a naphthene ring. This patent publication discloses that a fluid obtained by incorporating the above-mentioned alkane compound in perhydrogenated (.alpha.-methyl)styrene polymer, hydrindane compound or the like has a high traction coefficient. Further, Japanese Patent Laid Open No. 191797/1984 discloses a traction fluid containing an ester compound having a naphthene ring. It teaches that an ester obtained by the hydrogenation of the aromatic nucleus of dicyclohexyl cyclohexanedicarboxylate or dicyclohexyl phthalate is preferable as the traction fluid.
As mentioned above, there has been progress in recent years in the development of continuous variable-speed transmissions. The higher the traction coefficient of the traction fluid the larger the transmission force in the device. This contributes to a reduction in the size of the device with a corresponding reduction in exhaust gas, thereby reducing environmental pollution. Therefore, there is a demand for a fluid having a traction coefficient as high as possible. However, even the use of a traction fluid which exhibits the highest performance of all the currently commercially available fluids in such a traction drive device provides unsatisfactory performance with respect to the traction coefficient and economics. The traction fluid which has been proposed in Japanese Patent Publication No. 35763/1971 contains Santotrack.RTM., which is unsatisfactory with respect to performance and economics.